Process for liquefying a hydrocarbon-rich fraction

ABSTRACT

A process is proposed for liquefying a hydrocarbon-rich fraction (A), especially natural gas, by
     a) liquefying the hydrocarbon-rich fraction (A) against the coolant mixture of a cooling circuit,   b) compressing the coolant mixture in at least two stages (C 1,  C 2 ),   c) partially condensing (E 1 ) the compressed coolant mixture ( 2 ) at least downstream of the penultimate compressor stage (C 1 ),   d) compressing (C 2 ) the lower-boiling gas fraction ( 2′ ) obtained to the final pressure,   e) while cooling (E) the first higher-boiling liquid fraction ( 3 ) obtained, expanding it (a) to perform cooling and vaporizing it (E) against the hydrocarbon-rich fraction (A) to be cooled,   f) partially condensing (E 2 ) the coolant mixture fraction ( 4 ) compressed to the final pressure and separating the first lower-boiling gas fraction ( 5 ) obtained, after partial condensation (E), into a second lower-boiling gas fraction ( 7 ) and a second higher-boiling liquid fraction ( 6 ), and   g) liquefying and subcooling (E) the second lower-boiling gas fraction ( 7 ), sub-cooling (E) the second higher-boiling liquid fraction ( 6 ) and expanding the two fractions to different temperature levels to perform cooling (b, c), and partly heating and at least partly vaporizing them (E) against the hydrocarbon-rich fraction (A) to be cooled.   

     According to the invention, the composition of the coolant mixture is selected such that the final boiling point (dew point) of the second lower-boiling gas fraction ( 7 ) is at a lower temperature than the initial boiling point of the first higher-boiling liquid fraction ( 3 ).

SUMMARY OF THE INVENTION

The invention relates to a process for liquefying a hydrocarbon-rich fraction, especially natural gas, by

-   a) liquefying the hydrocarbon-rich fraction against the coolant     mixture of a cooling circuit, -   b) compressing the coolant mixture in at least two stages, -   c) partially condensing the compressed coolant mixture at least     downstream of the penultimate compressor stage, -   d) compressing the lower-boiling gas fraction obtained to the final     pressure, -   e) while cooling the first higher-boiling liquid fraction obtained,     expanding it to perform cooling and vaporizing it against the     hydrocarbon-rich fraction to be cooled, -   f) partially condensing the coolant mixture fraction compressed to     the final pressure and separating the first lower-boiling gas     fraction obtained, after partial condensation, into a second     lower-boiling gas fraction and a second higher-boiling liquid     fraction, and -   g) liquefying and subcooling the second lower-boiling gas fraction,     subcooling the second higher-boiling liquid fraction and expanding     the two fractions to different temperature levels to perform     cooling, and partly heating and at least partly vaporizing them     against the hydrocarbon-rich fraction to be cooled.

A process of this type for liquefying a hydrocarbon-rich fraction is known, for example, from German Patent Application 197 22 490. Such processes for liquefying hydrocarbon-rich fractions are employed, for example, in natural gas liquefaction plants with a liquefaction performance between 10 000 and 3 000 000 t/a of LNG. With the citation of German Patent Application 197 22 490, the content thereof is incorporated in its entirety into the disclosure-content of the present application.

In the liquefaction process described with reference to FIG. 2 of German Patent Application 197 22 490, the coolant mixture, in the course of the multistage compression thereof, is partially condensed after each compressor stage, typically against ambient air and/or water. A first liquid phase obtained at an intermediate stage is used to precool the hydrocarbon-rich fraction to be liquefied. The first gas obtained at the highest pressure is likewise partially condensed and separated into a second gas phase and a second liquid phase. The second gas phase is liquefied, decompressed and then partly vaporized in countercurrent to the hydrocarbon-rich fraction to be liquefied. The expanded second liquid phase, which is likewise present in biphasic form, is added to this partly vaporized coolant mixture stream. The aforementioned first liquid phase is, after the expansion thereof, added, likewise in biphasic form, to the aforementioned mixture stream of second gas phase and second liquid phase.

In practice, it is found that the mixing of two biphasic coolant streams has good technical controllability in what is called falling vaporization, as takes place, for example, on the outside of helically coiled heat exchangers. In the case of rising vaporization, as is typically implemented in plate heat exchangers, the biphasicity of the two streams to be mixed, however, can lead to problems. Since such mixing of biphasic streams in a plate heat exchanger is not technically controllable at present, the two biphasic streams are mixed in a vessel outside the heat exchanger and separated into a gas phase and a liquid phase. Since the liquid phase has to be fed in from this vessel to the heat exchanger with a gradient, conduction of at least one biphasic stream through an ascending line between the heat exchanger and the vessel is unavoidable. According to the load range, this can lead, however, to undesirable unstable flow forms and as a result to disrupted operation.

It is an object of the present invention to specify a process of the generic type for liquefying a hydrocarbon-rich fraction, which avoids the aforementioned disadvantages.

Upon further study of the specification and appended claims, other objects and advantages of the invention will become apparent.

To achieve these objects, a process is proposed for liquefying a hydrocarbon-rich fraction, characterized in that the composition of the coolant mixture is selected such that the final boiling point (dew point) of the second lower-boiling gas fraction is at a lower temperature than the initial boiling point of the first higher-boiling liquid fraction.

According to the invention, the first higher-boiling liquid fraction is not now added to the second lower-boiling gas fraction until after the complete vaporization thereof. This procedure dispenses with the provision of a biphasic riser line, described at the outset, between heat exchanger and vessel.

Further advantageous configurations of the process according to the invention for liquefying a hydrocarbon-rich fraction, which are the subjects of the independent claims, are characterized in that

-   -   the temperature difference between the final boiling point of         the second lower-boiling gas fraction and the initial boiling         point of the first higher-boiling liquid fraction is at least 5         K, preferably at least 10 K,     -   the second higher-boiling liquid fraction is vaporized         separately from the first higher-boiling liquid fraction and the         second lower-boiling gas fraction,     -   the first higher-boiling liquid fraction and the second         lower-boiling gas fraction are not combined until after they         have been vaporized with the second higher-boiling liquid         fraction,     -   at least a substream of the cooled second lower-boiling gas         fraction is added to the expanded second higher-boiling liquid         fraction,     -   expanded first higher-boiling liquid fraction and vaporized         second lower-boiling gas fraction are mixed outside the heat         exchanger(s) required for the heat exchange between the         hydrocarbon-rich fraction to be liquefied and the cooling         circuit, preferably in a separator, the vaporized second         lower-boiling gas fraction being supplied to the separator in         monophasic form,     -   the liquid fraction obtained in the partial condensation of the         coolant mixture fraction compressed to the final pressure         subcools the first higher-boiling liquid fraction.

BRIEF DESCRIPTION OF THE DRAWINGS

The process according to the invention for liquefying a hydrocarbon-rich fraction and further configurations thereof are explained in detail hereinafter with reference to the working example shown in the FIGURE.

The FIGURE shows a natural gas liquefaction process in which the natural gas to be liquefied is fed via line A to a heat exchanger E, liquefied against a coolant circuit and then drawn off via line B and sent to further use or storage thereof. The FIGURE does not show any pretreatment steps to be provided for the natural gas to be liquefied, or any removal of nitrogen and/or C₂₊ hydrocarbons to be provided, although such pretreatment steps may be included in the process according to the invention.

The coolant mixture to be compressed in the coolant circuit is supplied via line 1 to a first separator D1 which is connected upstream of the compressor unit C1/C2 and serves to remove condensate. The gas phase obtained at the top of the separator D1 is fed via line 1′ to the first compressor stage C1 and compressed to an intermediate pressure which is typically between 15 and 35 bar. Separator D1 acts as a safeguard for the first compression stage C1. In some operation modes, small amounts of liquid may collect within D1. This liquid can be withdrawn from D1 and optionally recycled.

The compressed coolant mixture is partially condensed in the heat exchanger E1 and fed via line 2 to a second separator D2. The first higher-boiling liquid fraction drawn off from the bottom of the separator D2 via line 3 is cooled in the heat exchanger E, decompressed to perform cooling in the valve a and then added via line 3′ to the coolant fraction in line 8, which will be discussed in more detail below. In an advantageous configuration of the invention, the expanded fraction 3′ and the coolant fraction 8 can also be mixed outside the heat exchanger E. In this case, a separator should be provided, to which the two aforementioned fractions are supplied, the coolant fraction 8 being supplied in monophasic form.

The gas phase drawn off from the separator D2 via line 2′ is compressed in the second compressor stage C2 to the desired final pressure, which is typically between 25 and 70 bar. The coolant mixture compressed to the final pressure is partially condensed in the heat exchanger E2 and fed via line 4 to a further separator D3.

The liquid phase obtained in the separator D3 is recycled via line 4′ upstream of the second separator D2. Appropriately, there is an exchange of heat between the liquid phases in lines 3 and 4′ in the heat exchanger E3, which preferably serves to subcool the liquid phase 3 drawn off from the bottom of the separator D2.

Via line 5, a first lower-boiling gas fraction is drawn off at the top of the separator D3. This is partially condensed in the heat exchanger E and then supplied to a further separator D4 via line 5′. A separation is effected therein into a second higher-boiling liquid fraction 6 and a second lower-boiling gas fraction 7. The second liquid fraction 6 is supplied to the heat exchanger E, subcooled therein and then expanded to perform cooling in the valve b. Via line sections 6′ and 10, the expanded liquid fraction is fed again to the heat exchanger E or passed through it.

The second gas fraction 7 obtained at the top of the separator D4 is likewise first liquefied and then subcooled in the heat exchanger E. After it has been drawn off from the heater exchanger E, this fraction is divided into two substreams 8 and 9. Both substreams are expanded to perform cooling in the valves c or d, respectively. While one substream is conducted via line 8 through the heat exchanger E and is vaporized in heat exchange against the hydrocarbon-rich stream to be liquefied, a further substream of the liquid fraction already mentioned can be added in the line 6′. This addition improves the controllability of temperature and cooling performance of stream 10, thus reducing the energy consumption, and/or serves to establish process conditions in the removal of nitrogen and/or C₂₊ hydrocarbons from the hydrocarbon-rich fraction A to be liquefied.

As shown by the FIGURE, the expanded second higher-boiling liquid fraction 6′ is vaporized separately from the expanded first higher-boiling liquid fraction 3′ and the expanded second lower-boiling gas fraction 8. This separate vaporization is effected in separate flow channels of the heat exchanger E. The aforementioned fractions are therefore not mixed until the hot end of the heat exchanger E, when these fractions are completely vaporized.

The separate vaporization leads to a slight increase in the energy consumption of the liquefaction process of up to 3%; however, this can be accepted in view of the improved operability of the liquefaction process.

The process according to the invention for liquefying a hydrocarbon-rich fraction now enables the unwanted rising biphasic flow, which was explained at the outset, outside the heat exchanger to be avoided. The disrupted operation caused to date by this biphasic flow can consequently be ruled out.

The entire disclosure[s] of all applications, patents and publications, cited herein and of corresponding German Application No. 10 2010 011 052.3, filed Mar. 11, 2010 are incorporated by reference herein.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples. 

1. A process for liquefying a hydrocarbon-rich fraction (A) comprising: a) liquefying hydrocarbon-rich fraction (A) against a coolant mixture of a cooling circuit, b) compressing said coolant mixture in at least two stages (C1, C2), c) partially condensing (E1) the compressed coolant mixture (2) at least downstream of the penultimate compressor stage (C1), thereby obtaining a lower-boiling gas fraction (2′) and a first higher-boiling liquid fraction (3), d) compressing (C2) said the lower-boiling gas fraction (2′) to a final pressure, e) cooling (E) said the first higher-boiling liquid fraction (3), expanding said first higher-boiling liquid fraction (3) (a) to perform cooling, and vaporizing said first higher-boiling liquid fraction (3) (E) against the hydrocarbon-rich fraction (A) to be cooled, f) partially condensing (E2) said coolant mixture fraction (4) compressed to the final pressure in b) to obtain a first lower-boiling gas fraction (5), and separating the first lower-boiling gas fraction (5), after partial condensation (E), into a second lower-boiling gas fraction (7) and a second higher-boiling liquid fraction (6), and g) liquefying and subcooling (E) said second lower-boiling gas fraction (7), subcooling (E) said second higher-boiling liquid fraction (6) and expanding the two fractions to different temperature levels to perform cooling (b, c), and partly heating and at least partly vaporizing the two fractions (E) against the hydrocarbon-rich fraction (A) to be cooled, wherein the composition of the coolant mixture is selected such that the final boiling point of said second lower-boiling gas fraction (7) is at a lower temperature than the initial boiling point of said first higher-boiling liquid fraction (3).
 2. A process according to claim 1, wherein the temperature difference between the final boiling point of said second lower-boiling gas fraction (7) and the initial boiling point of said the first higher-boiling liquid fraction (3) is at least 5 K.
 3. A process according to claim 2, wherein the temperature difference between the final boiling point of said second lower-boiling gas fraction (7) and the initial boiling point of said first higher-boiling liquid fraction (3) is at least 10 K.
 4. A process according to claim 1, wherein said second higher-boiling liquid fraction (6, 6′) is vaporized (E) separately from said first higher-boiling liquid fraction (3, 3′) and said second lower-boiling gas fraction (7).
 5. A process according to claim 4, wherein said first higher-boiling liquid fraction (3) and said second lower-boiling gas fraction (7) are not combined until after they have been vaporized with said second higher-boiling liquid fraction (6).
 6. A process according to claim 1, wherein at least a substream (9) of the cooled second lower-boiling gas fraction (7) is added to the expanded (b) second higher-boiling liquid fraction (6, 6′).
 7. A process according to claim 1, wherein expanded first higher-boiling liquid fraction (3, 3′) and vaporized second lower-boiling gas fraction (7, 8) are mixed outside the heat exchanger(s) (E) required for the heat exchange between the hydrocarbon-rich fraction (A) to be liquefied and the cooling circuit.
 8. A process according to claim 7, wherein the expanded first higher-boiling liquid fraction (3, 3′) and vaporized second lower-boiling gas fraction (7, 8) are mixed in a separator, and the vaporized second lower-boiling gas fraction (7, 8) is supplied to the separator in monophasic form.
 9. A process according to claim 1, wherein the liquid fraction (4′) obtained in the partial condensation (E2) of the coolant mixture fraction (4) compressed to the final pressure is used to subcool (E3) said first higher-boiling liquid fraction (3).
 10. A process according to claim 1, wherein said hydrocarbon-rich fraction (A) is natural gas. 